Rock cutting structures found in the prior art are commercially manufactured using a supporting structure which is formed by either casting or forging, and then is machined, and which is designed to support at least one rock cutting tooth, which tooth may be formed either as an integral part of the supporting structure, or as a separate detail which is pre-formed of a premium hard material and is designed to be joined to the supporting structure by being forced into an undersized retaining bore formed by and within the supporting structure to effect a tight interference fit therewith.
Various sequencing and scheduling strategies are resorted to in manufacturing in order to avoid heat treatments after the teeth are pressed into place, to avoid thermal relaxation of the induced stresses provided by the pressing operation. Such a reduction of induced stresses would result in an unacceptable reduction in the retention or holding force provided by the interference fitting operation.
The steels commonly used to produce rock bit cutting structures are graded which carburize or nitride readily, thus providing a relatively soft core with a hard wear resistant skin. To form this hard skin on the inner surface of the retention bores would seriously interfere with the installation of the teeth therein, and so generally the supporting structure is first carburized, then the carburized surface is machined away in the locations intended for retention bores, then heat treat hardened before the retention bores are machined.
The conventional three-cone rotary rock bit has, typically, from about 100 to about 300 inserted teeth, each of which is carefully selective fit to provide about 0.004 inch interference fit. Tests indicate that about 0.001 inch interference fit remains as stored stress within the assembly after the pressing, the rest being lost to shearing, galling, and yielding of the steel of the supporting structure.
The irregular heavy impact loads imposed upon the rock bit assembly during drilling tend to cause further yielding in the supporting structure with the subsequent enlargement of retaining bores, and, occasionally, the resultant loss of hard-metal rock cutting teeth within the well bore. Such a lost tooth is no longer operational as a cutting device against the rock, but does constitute a source of considerable damage or fracture to the remaining teeth in the rock bit. Serious damage can also occur as a result of a dislodged tooth becoming jammed between cones, or between a cone and the body of the rock bit, thereby interfering with the rotation and cutting action of the cones involved, and of the bit. When cone rotation ceases, a skidding action occurs between the well bore bottom and the cone or cones, a stopped cone quickly causes self-destruction of the cutting apparatus.
Additionally, such dislodged teeth, if left remaining loose in the bottom of a well bore, and not embedded in the well bore wall, present an equally destructive potential against fresh new rock bits introduced into the same well bore, consequently, it is all too frequently required to expend much time, effort, and a great deal of expense to "fish" debris from the remote well bore bottom in order to permit continued drilling activity, and thus to save the bore already accomplished.
The time and labor involved in raising and disassembling the drill line from a deep well, and then reversing the process to assemble and lower the drill line into the well again, is indeed a major expense item in an already monumentally expensive venture. This unwelcome procedure is required every time a rock bit must be changed, or debris must be "fished" from bore bottom. Anything that can be done to help avoid or to postpone a "trip" out of and back into a well bore, must be done, in the interest of survival in this high stakes endeavor.
Existing evidence indicates that the heavy hydraulic applied load applied to an insert type rock bit tooth for the purpose of forcing it into the retaining bore provided is sometimes instrumental in producing internal micro-cracking in the subject tooth resulting in the subsequent early failure of that tooth in service. Only by eliminating the pressing operation can the loss of inserts in service due to installation damage be eliminated.